chapter 12 wind, waves & shoreline currents bfrb pages 168-170 & 183-184

Chapter 12Chapter 12

Wind, Waves & Wind, Waves & Shoreline CurrentsShoreline CurrentsBFRB Pages 168-170 & BFRB Pages 168-170 &

183-184183-184

Erosion By WindErosion By Wind

• Dry ClimatesDry Climates (soil made of dry & loose sand, silt, & clay which are easily carried by wind)

• Mechanical weathering by ABRASIONABRASION (wind carries sediments and blows them against rocks – like sandpaper)– VENTIFACTSVENTIFACTS – “dents” in rocks from sand– Side of the rocks facing the wind are

usually worn smooth over time

VIF!!!!!

DEFLATIONDEFLATION

• Wind removes the top layer of soil, causing the ground surface to lower over time– The soil “DEFLATES” (like a

bicycle tire) and there are small depressions or “pits”

Deflation “pits”

SAND DUNESSAND DUNES

• Large piles of sand deposited by wind– Usually start piling up against rocks,

bushes, clumps of grass, etc…

• Side of the dune facing the wind (WINDWARD SIDE)(WINDWARD SIDE) is a long gentle slope

• Side opposite (LEEWARD SIDE)(LEEWARD SIDE) is a short, steep slope – also known as the “slip face” (like if you tried to walk down it you’d slip because it’s steep…)

windward

leeward

Remember Mrs. Frisby & the rats of Nimh? (Secret of Nimh)

The Rats of Nimh were asked to move Mrs. Frisby’s house to the LEEWARD

side of the hill so it would be protected!

Barchan Dunes - are crescent-shaped dunes with the points of the crescents

pointing in the downwind direction, and a curved slip face on the downwind side of

the dune.

Transverse (Longitudinal) Dunes - are large fields of dunes that resemble

sand ripples on a large scale.

Parabolic (also called blowout) Dunes - "U" shaped dunes with an open end facing upwind. They are common in

coastal areas.

Star Dunes - are dunes with several arms and variable slip face directions

that form in areas where there is abundant sand and variable wind

directions.

Waves & Waves & CurrentsCurrents

Causes of WavesCauses of Waves

•Wind blowing the surface of the water causing it to “pile up”

•HEIGHT depends on 2 things:–1 – length of time the wind blows

–2 – FETCHFETCH – distance the wind blows over the open water

Wave FeaturesWave Features

• Waves cause surface water to rise & fall in an orbital motion

• Top of the wave = CRESTCREST• Bottom of wave = TROUGHTROUGH• WAVE HEIGHTWAVE HEIGHT is the difference

between the crest and trough• WAVE LENGTHWAVE LENGTH is the distance from

one crest to the next crest

• PERIODPERIOD – the time it takes one complete wavelength to pass a point

• SPEEDSPEED = wavelength (dist) / period (time)

•Units are m/sec or ft/sec

Wave FeaturesWave Features

Waves in Shallow Waves in Shallow WaterWater

• BREAKERSBREAKERS – waves that enter shallow water, slow down & “break” – The bottom of the wave “scrapes”

along the bottom of ocean & slows down due to friction, this reduces its wavelength.

– The reduction in wavelength (horizontal) increases the wave height (vertical) – (Conservation of energy.)

– The crest of the wave continues forward at the same speed & falls over onto the beach. (The crest is not moving faster, it just appearsappears to move faster because the bottom part of the wave is moving slower!!!)

As waves enter shallow water, their length decreases which causes their height to

increase!

Waves in Shallow Waves in Shallow WaterWater

• Waves approach the shoreline at an angle

• In shallow water, the part of the wave that “hits” the shallow shoreline first, slows down (due to friction) and the part of the wave that isn’t hitting the shoreline keeps moving at the same speed.

• The wave crest direction “bends” due to the difference in speed.

• Shoreline waves REFRACTREFRACT & hit the shore almostalmost parallel to shore BUT NOT PARALLEL!!!!!

CURRENTSCURRENTS• SWASHSWASH – water moving ONTO the beach• BACKWASHBACKWASH – water moving back off

beach – Pulls sand from the beach into the water– UNDERTOWUNDERTOW – a strong backwash – very

dangerous for small children!

Ocean

SWASH

BACKWASH

• VIF!!! LONGSHORE CURRENTSVIF!!! LONGSHORE CURRENTS – move parallel to shoreline & carry sand down the beach (erosion)– Ever notice how when you get out of the

ocean after swimming you’re farther down the beach from your towel?!?!

• RIP CURRENTSRIP CURRENTS – very strong currents that move OUTWARD from the shore– Extremely dangerous – can pull you from

shore!• The WORST thing to do if caught in a riptide is to

panic & swim directly to shore! The current is too strong & you will fatigue and drown! Swim diagonally across the current to safety!

CURRENTSCURRENTS

Longshore Current:

transportation of sediments parallel to the beach…this is the current that can transport you parallel to the beach as well!

SHORELINSHORELINE E

FEATURESFEATURES

• Waves easily erode soft rocks found Waves easily erode soft rocks found along deep water shorelinesalong deep water shorelines

• Sea Cliffs, Sea Arches, Sea Caves, and Sea Cliffs, Sea Arches, Sea Caves, and Sea Stacks are commonly found off the Sea Stacks are commonly found off the coast of Maine, California, Oregon, coast of Maine, California, Oregon, Hawaii, Canada and Australia.Hawaii, Canada and Australia.

Haystack Rock, OR

• Longshore currents deposit large Longshore currents deposit large amounts of sediment over time.amounts of sediment over time.

• A A SANDBARSANDBAR is formed when ocean is formed when ocean currents deposit sand near the currents deposit sand near the shore. Sandbars may take many shore. Sandbars may take many different forms…different forms…

• 1) BARRIER ISLANDS1) BARRIER ISLANDS are sandbars are sandbars parallel to shore (Fire Island, NY & parallel to shore (Fire Island, NY & the Outer Banks, NC)the Outer Banks, NC)– Barrier islands protect the shoreline Barrier islands protect the shoreline

from the full strength of wavesfrom the full strength of waves– These shoreline features are NOT These shoreline features are NOT

permanent! They are constantly being permanent! They are constantly being built up built up andand destroyed by waves! destroyed by waves!

– Most of the Eastern Coast of the USA Most of the Eastern Coast of the USA has Barrier Islands and flat sandy has Barrier Islands and flat sandy beaches.beaches.

Barrier Islands… think

Fire Island, Long Island and Outer Banks, No. Carolina

Bay: indentation of shoreline

Think – San Francisco Bay…

Spit: sandbar that is connected to land on one side of a bay

Baymouth Bar – a sandbar that completely closes access to a bay, thus closing it off from the main body of water

Lagoons – a – a shallow, shallow,

protected area protected area of water located of water located

landward from landward from a sandbar.a sandbar.

Jetties and

Groins